FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
"A Coating to Reduce Near Infrared Radiation, A Method of Manufacturing
And Applying The Coating."
Tata Chemicals Limited, an Indian company of Bombay House, 24 Homi Mody Street, Mumbai -1
The following specification particularly describes the invention and the manner in which it is to be performed.

The invention relates to a coating for substrates and to a method of manufacturing the same. More particularly, the invention relates to a coating capable of reducing near infrared radiation and a method of manufacturing and applying such a coating.
Energy savings is a growing concern and transparent glazing systems are progressively being used in the exteriors of commercial buildings that allow considerable amount of solar energy into the interiors. Thin-film coatings on glass are commonly utilized to provide specific energy attenuation and light transmittance properties. Additionally, such coatings also called as low-emissivity coatings (low e coatings) are designed to reduce transmittance of near infrared (NIR) radiation, while allowing sufficient visible light for illumination within the interiors and comfortable visibility through the glass walls/windows. The coated articles are often utilized singularly, or in combination with other coated articles, to form a glazing.
The attributes of a coated glass substrate are dependent upon the specific coatings applied to the glass substrate. The coating compositions and thicknesses impart energy absorption and/or reflection and light transmittance properties while also affecting the spectral properties. Desired attributes may be obtainable by adjusting the compositions or thicknesses of the coating layer or layers. However, adjustments to enhance a specific property may adversely affect other transmittance or spectral properties of the coated glass article. Obtaining desired spectral properties is often difficult when trying to combine specific energy absorption and light transmittance properties in a coated glass article. Further, finding a suitable additive material for coatings having good NIR radiation absorption characteristics with sufficient optical transmittance in the visible region is challenging. Especially since the additive material need to be stable that do not
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"degrade under harsh conditions unlike organic dyes and are compatible with the coating matrix.
Various methods for obtaining energy saving solar control coatings on glass are known. Most rely on sputter depositing, pyrolytipally depositing or other vacuum depositing techniques for making a single layer or multilayer spectrally selective solar control coatings consisting essentially of alternating layers of several metals and dielectric metal oxides. These types of coatings presently constitute the major part of the commercially available solar coatings. Alternatively, solar control coatings have also been made by deposition of NIR absorbing materials blended with polymeric matrices.
Some anti-reflective coatings for solar control applications have sputter deposited layers of dielectric materials such as silicon zirconium nitride and silicon oxide with interspersed metallic layers of silver. Other, coating systems describe laminated glass with interlayer polymeric film with dispersed ultra fine particles of antimony doped tin oxide and indium doped tin oxide for heat insulation, ultraviolet ray absorption along with sufficient radio transmittance unlike the glazing consisting of continuous conductive metallic layers that shield radio waves.
Other solar control films comprise vapour deposited aluminium coats that transmit light, interposed between a vinyl stratum and a polyester stratum. The film can be adhered by pressing the surface of its vinyl stratum against the window glasses in dry or wet condition. Other solutions include near infrared dyes of divalent immonium salts that can be applied for solar control glazing or liquid coatings of NIR absorbing dyes, a carrier Vehicle and solvent system.
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Some coating systems describe glazing panels comprising first and second glass sheets in spaced relation with first glass sheet having an oxide coated surface on the exterior and a gold coating on the surface which faces the second glass sheet. Other coating systems disclose pyrolitically deposited silica film on glass to prevent migration of alkali metal ions to the surface.
Examples of such coatings may be found in US 7090921, US5.830568, US6329061, US 5830568, US 5686639, US 20050123746, US 4487197 and VUS 5165972.
However, the coating systems comprising of most metal layers are subjected to corrosion and deterioration due to migration of alkali metal ions whereas coatings of continuous metal layer reflect visible light and thus need to be managed with additional antireflection coating systems over it. Moreover, continuous conductive metal coatings also considerably reduce transmittance of radio waves through them. Hence, a nonconductive coating with reduced reflectivity in the visible spectrum of light will be an ideal alternative. Coatings based on dispersed NIR absorbing agent dispersed in polymeric matrix serve this purpose but lack the durability due to poor abrasion resistance and degradation due to incident solar radiation over prolonged use.
It would be desirable to provide a coating that addresses at least some of the limitations of existing coatings and has a longer life and ease of application.
Summary of the Invention:
The present invention relates to a coating with near infrared absorbing properties for solar control applications and a process of making the same. More particularly, the
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invention provides for a coating comprising of alkoxysilane, alkylsubstituted alkoxysilane, near infrared absorbing dispersed flat 2D gold nanoparticles and a polymeric dispersing agent in a solvent system, and a method of manufacturing and applying the same.
Detailed Description of Invention
To promote an understanding of the principles of the invention, reference will be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated suspension system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
The invention provides for a coating, a method of making the same, a method of applying the coating on a glass surface and a glass surface so produced. The invention provides a coating with NIR radiation absorption characteristics for application on glass substrates. The coating includes a sol prepared using an alkoxysilane in combination with alkylsubstituted alkoxysilane.
The coating sol in accordance with an embodiment of the invention preferably has pH in the range 1 - 4 and comprises of flat gold nanoparticles along with polyvinylpyrrolidone (PVP). PVP molecules are bound to the surface of the gold nanoparticles, i.e. capped to the gold nanoparticles. The PVP molecules by way of
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capping the gold nanoparticles prevent them from aggregating and facilitate their stable dispersion. The coating is sufficiently abrasion resistant and durable enough for prolonged use as it is based on hard silica matrix and the use of gold particles as NIR absorbing material.
The invention, in accordance with an embodiment provides for a coating of tetraalkoxysilane in combination with alkyltrialkoxysilane, near infrared radiation absorbing flat gold particles and a polymeric dispersing as well as capping agent. The coating is preferably a sol formulation and is useful for providing transparent and hard coatings on glass and related substrates.
The gold particles are preferably flat and of triangular or hexagonal shapes having NIR absorbing characteristics. The gold particles also preferably have edge lengths in the range of 50 nm to 800 nm with thickness in the range of 8 nm to 35 nm. It is preferred that the gold particles are capped by a dispersing agent.
In accordance with an embodiment, the coating preferably contains 1-10 wt% gold particles with respect to equivalent SiC>2 or 0.06-0.8 wt% with respect to the final weight of the coating sol.
Any suitable polymeric dispersing agent may be used for the coating. In accordance with a preferred embodiment, the polymeric dispersing and capping agent for use in the coating may be PVP that has desirable functional groups to bind with the particles surface and sufficient compatibility with the coating medium to form a stable and concentrated dispersion. In accordance with a preferred embodiment approximately 2 - 5 wt % of a polymeric dispersing agent such as PVP may be used with respect to the coating. It is also preferred that PVP of average molecular weight in the range of 58,000
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- 1,300,000 is used that does not cause any opaqueness to the coating or distort the optical quality of the coating.
It is desirable to keep the pH value of the coating between 1 and 4.0, as the -Si-O-Si- condensation reaction is slow resulting in increased shelf life of coating.
The coating in accordance with an embodiment includes tetraalkoxysilane and alkyltrialkoxysilanes.
The invention also provides for a method of making the coating in accordance with the invention. The method comprises preparation of a coating from an alkoxysilane and alkylsubstituted alkoxysilane containing flat gold particles suitable for deposition on glass and related substrates.
The hardness of the coatings is achieved by controlled heat-treatment process causing elimination of organic matter and densification of the silica matrix.
The gold particles are first capped with a suitable polymeric dispersing agent preferably containing surface active nitrogen moiety such as PVP and its analogues in an' aqueous medium, followed by concentrating the solution and adding it to an alcoholic solution of tetraalkoxysilane and alkyltrialkoxysilane (precursors of silica). It is also preferred that the PVP capped gold particles are concentrated to an extent such that the remaining water (-0.5-2 mol/alkoxo group) could be utilized for the hydrolysis-condensation reactions of the alkoxide groups of the precursors for the formation of the silica sol.
In accordance with an embodiment the invention provides for a coating using tetraalkoxysilane and alkyltrialkoxysilane. Accordingly the invention provides a process of making a coating which comprises mixing one tetraalkoxysilane (TAS) and one
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alkyltrialkoxysilane (ATAS), wherein mole ratios of TAS and ATAS are in the range of 1:1 to 4:1 in one or a mixture of organic solvents. The PVP capped gold particles, containing small amount of water, are added to the above mentioned mixture. The resultant mixture is preferably stirred for 30-240 min to obtain a clear solution.
Tetraalkoxysilanes include tetramethyl oithosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate and tetrabutyl orthosilicate.
Alkyltrialkoxysilanes include methyltrimethoxysilanes, ethyltrimethoxysilanes, ethyltriethoxysilanes and methyltriethoxysilanes.
The organic solvents used may be any suitable solvent such as n-propanol, n-butanol, ethanol, methanol mixed with HCl in the range of 10"3 to 10"4 mole per mole of alkoxy group.
The silica network is preferably generated in situ through the hydrolysis-condensation reactions of tetraalkoxysilane (TAS) in presence of alkyltrialkoxysilane (ATAS). In a further embodiment of the present invention, silica nanoparticles are expected to be generated that further grow as silica network in sol stage.
Methods of preparing gold particles suitable for use in the coating have been described in articles co-published by authors (Ref: S. Shiv Shankar, Akhilesh Rai, Absar Ahmad and Murali Sastry, Chem. Mater. 2005, 17, 566-572) and the same are incorporated here in entirety. The documents describe a method for the synthesis of flat 2D gold nanoparticles from aqueous chloroauric acid solution using natural reducing agents such as leaf extracts of lemongrass plant (cymbopogon flexosus, cymbopogon citratus). The gold nanoparticles synthesized using the lemongrass leaf extract had a majority of flat 2D nanoparticles with mainly triangular and some hexagonal morphology
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apart from the usual spherical nanoparticles. The flat 2D gold nanoparticles have a large absorbance in the NIR region with a peak position proportional to their size.
The gold nanoparticle solutions so formed are subjected to centrifugation followed by discarding a major part of supernatant solution. The concentrated solution is then ultrasonicated for few minutes prior to addition of the polymeric dispersing agent such as PVP to cap the Au nanoparticles. For example, 25 gm of 10 wt % PVP (average molecular weight 3,60,000) in methanol is added to a concentrated gold nanoparticles solution obtained by centrifuging 3 L of 10" M gold nanoparticles solution synthesized using lemongrass leaf extract and stirred vigorously for 2 hrs. Later, the PVP capped gold ' nanoparticles solution is further concentrated by rotary vacuum evaporation till 5 - 8 gm of residue is left. Also, as an example, 32 gm of 10 wt % PVP (average molecular weight 1,300,000) in methanol is added to concentrated gold nanoparticles solution and stirred vigorously for 2 hrs followed by rotary vacuum evaporation till 12 - 20 gm of residue is left. The concentrated PVP capped gold nanoparticles have large percentage of flat gold nanoparticles of triangular/hexagonal shapes and have large NIR absorbance.
By way of example, and in accordance with an embodiment, in a separate container 16.1 gm of tetraethoxysilane (TEOS), 16.1 gm of ethyltriethoxysilane (ETES) and 14 gm of 1-butanol were mixed and stirred for 15 min, followed by addition of 0.5 gm of cone. HC1 (35.5%) and 6 gm of methanol under the stirring condition. To this solution, PVP-capped- NIR absorbing gold nanoparticles (12-20 gm) dissolved in 30 gm methanol and 40 gm 1-propanol was added under stirring condition at room temperature. Stirring was then continued for 180 min at room temperature (25±2 °C) for allowing
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hydrolysis of the silica precursors (TEOS and ETES). This hydrolyzed silica solution containing PVP capped gold nanoparticles provides the coating.
By way of another example, and in accordance with another embodiment of the invention, in a separate container 10 gm of tetraethoxysilane (TEOS), 7 gm of ethyltriethoxysilane (ETES) and 7 gm of 1-butanol were mixed and stirred for 15 min, followed by addition of 0.1 gm of cone. HCl (35.5%) and 3 gm of methanol under the stirring condition. To this solution, PVP-capped- NIR absorbing gold nanoparticles (5-8 gm) dissolved in 17 gm methanol and 23 gm 1-propanol was added under stirring condition at room temperature. Stirring was then continued for 60 min at room temperature (25±2 °C) for allowing hydrolysis of the silica precursors. This hydrolyzed silica solution containing PVP capped gold nanoparticles provides the coating.
In accordance with an embodiment of the invention, the coating is applied by vertically dipping and lifting the substrate from the coating sol at controlled rates of withdrawal and further heat treating the coated glass to densify the matrix and eliminate the organic matter, such as the capping /dispersing agent PVP, alkyl groups attached to Si and unhydrolyzed alkoxy groups if any, by decomposition and for achieving sufficient hardness.
The heat treatment is done in a controlled manner, preferably in the range 250-500°C, to enhance densification of the silica matrix by promoting cross-linking/condensation within the matrix and decomposition of the organic matter while minimizing the deformation of the embedded flat gold particles.
It is preferred that drying at a temperature in the range of 50 to 70°C is carried out after dipping the substrate in coating. This is followed by heat treatment at a temperature
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in the range of 250 - 500°C for a period of 5 to 10 hours or preferably 275-325°C for 5-10 h followed by 450° forlO to 30 min and optionally at 500 °C for 5-10 min for further increase in coating hardness.
In another embodiment of the present invention, the coating can be used more than 30 days if aged sol is- warmed at around 30-40 °C for few min prior to coating deposition.
In a yet further embodiment of the present invention, the coating is applied on glass substrate or related substrates by dipping or spin coating technique or other similar techniques employed for sol-gel coatings.
In yet another embodiment of the present invention, the organic part of the coating is burnt during the said heat treatment process leaving a continuous glass like matrix of silica with uniformly dispersed flat gold particles for imparting sufficient scratch and abrasion resistance to the coating.
In yet another embodiment of the present invention, the post heat treated coating contains 1-10 wt% gold nanoparticles with respect to equivalent SiC>2.
In still yet another embodiment of the present invention, the coating can be applied on glass or other related substrates.
The heat treated coating so applied meets the specifications, such as: (i) coating after heat treatment is faintly coloured but transparent, (ii) coated materials transmit nearly -40-50 % NIR radiation within the range 800 nm to 2000 nm and transmit - 60 -70 % light in the visible region, (iii) hardness value of the coatings is about 6H or greater (ASTM D 3363), (iv) the coating passes the abrasion test in accordance with U.S. specification MIL-F-48616 with an eraser having specification MIL-E-12397B.
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In a still further embodiment of the present invention, the heat treated coating thickness is in the range of 0.4 to 1.2 /im.
By way of example, specifics of a coating application are described. Prior to coatings deposition glass substrate is first cleaned with detergent, followed by washing with tap water and rinsing with distilled water and ethanol and finally 5 min in warm isopropanol. Cleaned glass substrates is immersed into the coating and withdrawn at a rate of 10 cm/min to 20 cm/min from the sol perpendicular to the solution surface. The deposited coatings are then dried at 60-70°C for 60 min and finally heat treated at a temperature in the range of 275-375°C for a period of 5 to 10 hours or preferably at 275-325°C for 5-10 h followed by heat treatment at 450° for 10 to 30 min and optionally further at 500 °C for 5-10 min for additional hardness. The thicknesses of the heat treated coatings so obtained are in the range 0.4-1.2 [im
The resultant coatings of 0.4-1.2 fim in thickness are optically transparent and showed good abrasion property. The cross-cut scotch tape test (following DIN 53151 or ASTM D 3359 specification) of the coatings deposited on show no damage of the coatings. Hardness of the coating is about 6H to more than 6H. Coated materials transmit nearly ~ 40 - 50 % NIR radiation within the range 800 nm to 2000 nm and transmit ~ 60 -70 % light in the visible region (400-800 nm).
The protective thin film preferably has a thickness that is in between 0.4-1.2 /xm and most preferably between 0.5-0.7 fim.
A protective film in accordance with the present invention provides good abrasion and scratch resistance.
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In an example of coating derived from a sol consisting of about 7 wt% equivalent of silica and nearly 0.6 wt % of gold nanoparticles with respect to the final weight of the sol; following characteristic properties were obtained after heat treatment:
• Appearance : optically clear, bluish tint
• Thickness : 550 nm
• Gold nanoparticle in the film : - 8 wt % with respect to equiv.
wt% Si02
• Hardness (ASTM D 3363) :~6H
• Adhesion (cross cut; ASTM D.3359): No peal off the coating material.
The high hardness of the coatings is 6H plus (ASTM D 3363) due to the careful thermal densification of the matrix SiC>2. After deposition of coatings, coated materials transmit nearly ~ 40 - 50 % NIR radiation within the range 800 nm to 2000 nm and transmit - 60 - 70 % light in the visible region.
The optical absorption characteristics (visible to near infrared) of the above representative film is given below in a tabular form (Table 1). It can be seen that the film has maximum transmission of -65% in the range 600-700 nm (visible) whereas the average NIR transmission (900-1500 nm) is of the order of 48%.
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Tablel
1 able showing the % transmission values at different wavelengths in the visible and NIR region for a representative coating as mentioned above.

Wavelength (nm) % Transmission Wavelength (nm) % Transmission
400 53 1225 47.5
425 57.5 1250 47.5
450 59.5 . 1275 47.5
475 63 1300 47
500 63.5 1325 47
525 58' 1350 47
550 56 1375 46.5
575 61 1400 46.5
600 64 1425 46.5
625 64.5 1450 46.5
650 65 1475 47
675 65.5 1500 47
700 65 1525 47.5
725 63 • 1550 48
750 61 1575 48.5
775 59.5 1600 49
800 58.5 1625 49
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825 57.5 1650 50.5
850 56.5 1675 51
875 56 1700 52
900 55 1725 52.5
925 54 1750 53.5
950 52.5 1775 54
975 51 1800 55
1000 50 1825 56
1025 49 1850 56.5
1050 49 1875 57.5
1075 48 1900 58.5
1100 48 1925 59
1125 48 1950 59.5
1150 48 1975 60
1175 48 2000 61
1200 48 2025 61.5
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We claim:
1. A coating to reduce near infrared radiation for substrates comprising:
- an alkoxysilane,
- an alkylsubstituted alkoxysilane,
. - gold particles capable of near infrared radiation absorption; and
- a polymeric dispersing and capping agent.
2. A coating as claimed in claim 1 comprising tetraalkoxysilane and alkyltrialkoxysilane.
3. A coating as claimed in claim 1 comprising, monoalkylsubstituted trialkoxysilane or dialkylsubstituted dialkoxysilane.
4. A coating as claimed in claim 1 comprising tetraethoxysilane and ethyltriethoxysilane.
5. A coating as claimed in claim 2 wherein the mole ratios of tetraalkoxysilane and alkyltrialkoxysilane are in the range of 1:1 to 4:1.
6. A coating as claimed in claim 2 wherein tetraalkoxysilanes include tetramethylorthosilicate, tetraethylorthosilicate, tetrapropylorthosilicate and tetrabutylorthosilicate.
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7. A coating as claimed in claim 2 wherein alkyltrialkoxysilanes include methyltrimethoxysilanes, ethyltrimethoxysilanes, ethyltriethoxysilanes and methyltriethoxysilanes.
8. A coating as claimed in claim 1 wherein the gold particles is approximately 0,06-0.8% wt with respect to the final weight of the coating sol.
9. A coating as claimed in claim 1 wherein the gold particles are triangular or hexagonal in shape.
10. A coating as claimed in claim 9 wherein the gold particles are flat.
11. A coating as claimed in claim 1 wherein the thickness of the gold particles is in the range of 8 to 35nm.
12. A coating as claimed in claim 1 wherein the edge length of the gold particles is in the range of 50 nm to 800nm,
13. A coating as claimed in claim 1 wherein the polymeric dispersing and capping agent is polyvinylpyrrolidone.
14. A method of manufacturing a coating as claimed in claim 1 to reduce transmission in near infrared radiation comprising:
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- mixing of alkoxysilane and alkylsubstituted alkoxysilane in a solvent;
- hydrolyzing and condensing alkoxy groups of alkoxysilane and alkylsubstituted alkoxysilane by adding HC1 dissolved in alcohols with water;
- adding capped and concentrated gold particles to form coating

15. A method as claimed in claim 14 wherein solvents include n-propanol, n-butanol, ethanol and methanol alone or in combination with another solvent.
16. A method as claimed in claim 14 wherein HC1 is in the range of 10"" to 10"4 mole per mole of alkoxy group.
17. A method of applying a coating as claimed in claim 1 to a substrate comprising:

- dipping the substrate in the solution;
- withdrawing the substrate from the solution at a controlled rate;
- heat treating the coated substrate to densify the matrix and provide a coated substrate with near infrared absorbing properties.
18. A method of applying a coating as claimed in claim 17 wherein heat treatment
is done in the range of 275 to 375 degrees and preferably in the range of 275
to 325 degrees for five to ten hours.
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19. A method as claimed in claim 18 wherein additional heat treatment in the range of 450 to 500 degrees is carried out for ten to thirty minutes.
20. A method as claimed in claim 17 wherein the thickness of the coating is 0.4 to 1.2 urn.
21. A coating substantially as herein described with reference to the
accompanying description.
22. A method of manufacturing a coating substantially as herein described with reference to the accompanying description.
23. A method of applying a coating substantially as herein described with reference to the accompanying description.
Dated this 17th day of April 2007

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Abstract
The present invention discloses a sol formulation and method of applying it on a glass sheet to form a hard coating for solar control applications. The sol formulation consists of alkoxysilane, alkylsubstituted alkoxysilane, dispersed near infrared radiation absorbing flat gold particles and a polymeric dispersing agent. ,
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